The present invention relates to techniques for finishing a surface of a part. More particularly, the present invention relates to improved techniques for obtaining a quality finish on a silicon part at a relatively low cost.
Those involved in the manufacture of parts have long recognized the benefits to improved surface finishing techniques. The market for many parts could be increased if the part could be provided with a better surface finish or with a quality surface finish at a lower cost.
Different materials obviously require different equipment and techniques for obtaining the desired surface finish. Within the past decade, there has been an increased emphasis upon parts formed from materials not commonly used in the part manufacturing business. Those skilled in the design of equipment have long recognized the benefits of a part formed from a silicon material, since silicon has unusually high quality characteristics which are highly desirable for certain applications. In spite of the limitations associated with providing a desired finish on a silicon part, silicon parts have been increasing in popularity, particularly for unique applications in the electronics, telecommunications, and space industries.
Manufacturers have long been able to make a thin slice from a silicon block, thereby forming a desired number of silicon wafers. Conventional technology thus is able to grow a silicon crystal, and from that crystal obtain sliced silicon wafers. In general, however, it has been considered impractical to provide techniques to provide a desired surface finish on silicon parts other than flat planar wafers, primarily because of the tendency of a silicon part to shatter when mechanical forces are applied to the surface during the finishing operation.
Silicon parts have been manufactured and the surface of such parts conventionally machined with a conventional fine grit wheel. This finishing technique produces a surface which is acceptable for some applications, but does not produce a highly smooth surface, with minimum surface and subsurface damage, to meet the desires of many users. Accordingly, the market for materials formed from silicon, fused silica, silicon carbide, and similar materials has been limited due to the difficulty and cost associated with providing a desired finish on the silicon part. The term xe2x80x9csilicon partxe2x80x9d as used herein means a part formed substantially from one or more of silicon, fused silicon, and/or silica carbide.
While many applications conceivably could benefit from improvements in both the part machining and finishing techniques, optical elements constitute a class of goods wherein finishing techniques have been most beneficial. In the sequencing of machining, lapping, and polishing an optical element, it is machining that proceeds most rapidly but usually results in a surface of low quality by optical standards. Subsequent lapping and polishing constitute a large amount of the total fabrication time, but significantly enhance the quality of the machined surface. Finishing techniques applied to optical elements thus evidence the importance of a quality machined surface to reduce the time required for lapping and polishing, and thereby reduce the time required to manufacture the finished component.
In order to perform their desired function, most silicon parts cannot practically be used as wafers, since more complex geometries are generally required. Using conventional technology, three dimensional silicon parts have been manufactured and surfaces finished within the ballpark of from 20 to 50 RMS. Prior art techniques used to finish a silicon part generally include a two body method (part rotates and/or reciprocates; wheel rotates) which uses a no pitch abrasive, and a three body method (part rotates, wheel rotates, pitch is used) which uses pitch and a rotating grinding action. Both of these methods result in subsurface damage to the silicon part, and are time consuming.
One of the primary problems with these prior art techniques is that the process of obtaining this desired finish results in the fracture of a very high percentage of parts. It is not uncommon in the process of seeking to obtain a desired finish of a silicon part to fully machine then start the finishing process with 100 parts, with the eventual hope of obtaining 5 useful finished pieces. Since the other 95 being ruined in the finishing process, these techniques are very time consuming to increase the acceptable part vote and, regardless of the time spent, a very high percentage of the machined parts fracture during the finishing process. At this high cost, surface finishing of silicon parts in the range of from 20 to 50 RMS have been obtained, but higher quality surface finishes in the range of 9 RM and less had been considered impractical.
Various articles have been written with respect to the machining of silicon and glass in the ductile mode, such as Puttick, K. E., Shabid, M. A. and Hosseini, M. M., xe2x80x9cSize Effects in Abrasion of Brittle Materialsxe2x80x9d, J. Phys. D: Appl. Polys., Vol. 12, 195-202, 1979; Puttick, K. E., et al., xe2x80x9cSingle-Point diamond Machining of Glassesxe2x80x9d, Proc. R. Soc. London A 426. 19-30, 1989; Puttick, K. E., et al., xe2x80x9cLetter to the Editorxe2x80x94Surface Damage in Nanomachined Siliconxe2x80x9d, Sem. Cond. Sci. Technol. 7, 255-259, 1992; and Puttick, K. E., et al., xe2x80x9cTransmission Electron Microscopy of Nanomachined Silicon Crystalsxe2x80x9d, Philosophical Magazine, Vol. 69, No. 1, 91-103, 1994). Danyluk, S. and Reaves, R., xe2x80x9cInfluence of Fluids on the Abrasion of Silicon by Diamondxe2x80x9d, Wear 77 (1982) 81-87 and Danyluk, S., xe2x80x9cSurface Property Modification of Siliconxe2x80x9d, NASA-CR-173952, January 1984 relate to the effect of the coolant formulation on the hardness of silicon surfaces. Kersian, M., et al., xe2x80x9cUltraprecision Grinding and Single Point Diamond Turning of Silicon Wafers and Their Characterizationxe2x80x9d, Proc. ASPE Spring Topical Meeting on Silicon Machining, April 1998; Hashimoto, H. and Imai, Kl, xe2x80x9cEpistemology and Abduction in Shear (Ductile)-Mode Grinding of Brittle Materialsxe2x80x9d, Proc. ASPE Spring Topical Meeting on Silicon Machining, April 1998; and Ball, M. J., et al., xe2x80x9cCost Effective Edge Machining of Silicon Wafers to Minimize the polishing Processxe2x80x9d, Proc. ASPE Spring Topical Meeting on Silicon Machining, April 1998 teach that material removal should be done by many shallow cuts if damage is to be minimized. One reference suggests that the coolant may influence the nature of the surface being cut, although Chargin, D., xe2x80x9cCutting Fluid Study for Single Crystal Siliconxe2x80x9d, Proc. ASPE Spring Topical Meeting on Silicon Machining, April 1998 indicates that little benefit of coolants over deionized water obtained when SPDT is the method of material removal.
Kersian, M. et al. suggests that SSD may be machined in the range of 1 to 3.5 microns, while Ball, M. J. et al. suggests a range of from 2 to 5 microns. Using a 600 and 400 grit sample, Ball, M. J. et al. suggests subsurface damage level of 7 to 12 and 10 to 15 microns, respectively.
The disadvantages of the prior art are overcome by the present invention, and an improved method of finishing a silicon part utilizing a rotatable grinding wheel and one or more grip material is hereinafter disclosed.
The method of finishing a silicon composition part according to the present invention may be used to significantly reduce the amount of time to manufacture a silicon part, but also to significantly increase the percentage of parts which may be successfully finished without ruining the part. Finally, the present invention is particularly useful for finishing a silicon part to obtain a surface finish significantly below that achieved using prior art techniques.
The method according to the present invention uses a rotatable grinding wheel having diamond particles and a bonding materials. The method involves dressing the rotatable grinding wheel to form a grinding wheel surface having a plurality of diamond particles forming a substantially uniform particle grinding diameter. Thereafter, the bonding material is removed from the grinding wheel surface without significant removal of the plurality of diamond particles. An enhanced lubricity material, such a graphite, may then be applied to the grinding wheel. The operator may thereafter grind a surface of the silicon material part with the rotatable grinding wheel, and finally finish the ground surface of the silicon material part with one or more grit materials.
In a preferred embodiment, a plurality of grit materials are used which vary from about 200 grit to less than 800 grit. The silicon composition part may be cooled with a plurality of cooling lines both when grinding the silicon material part with the grinding wheel and when finishing the silicon material part with the one or more grit materials. The techniques of the present invention may be used to obtain a surface finish on a silicon part which has a value of less that 9 RMS.
It is an object of the present invention to provide improved techniques for removing subsurface damage and providing a high quality finish and a flatness to a finished surface of a silicon part which is significantly improved over the prior art, and to provide such a finish at a reduced time compared to prior art techniques.
It is another object of the present invention to significantly reduce or eliminate micro-subsurface damage to a silicon part during the finishing operation.
Still another object of the invention is to achieve a finish on a silicon part which may be conventionally machined, with that finish being achieved with both a minimal amount of time and no substantial subsurface damage.
It is a feature of the present invention to significantly reduce the time required to turn and lap a machined silicon part.
It is also a feature of this invention that the silicon part is cooled with a plurality of cooling lines both when grinding the silicon part with the grinding wheel and when finishing the silicon part with one or more grit materials.
It is a further feature of the invention to provide graphite as the enhanced lubricity material applied to the grinding wheel.
Still another feature of the invention is that the part is covered while grinding the surface of the part using an overhead cover having a surface area at least four times the maximum nominal diameter of the part.
It is an advantage of the present invention that many of the techniques and methods as taught herein for forming a silicon part with a desired surface finish may be performed in a conventional manner by changing the coolant and/or checking the machining or finishing the parts for defects.
These and further objects, features, and advantages of the present invention will become apparent from the following detailed description, wherein reference is made to the figures in the accompanying drawings.